I'm working (still!) on a customer quilt and needed to figure out a design for the setting triangles. Out comes the handy dandy plexiglad sheet and a dry erase marker.
I had to figure something out that wouldn't require me to quilt it too densely, but would still look great. Feathers tend to fit that bill nicely. It's a bot of an odd shape, but the echo around the black helped.
Above, the design itself, off the quilt. Below, it's all quilted and worked great. You can use plexiglas, clear vinyl or even a page protector or laminating sheet to figure out designs. Just make sure to devote one particular side for the drawing so marks don't accidentally transfer and make sure that you don't go off the edge onto the quilt!
I also use the plexiglass as an oversized ruler for squaring quilts up and sometimes for marking out the size of a whole cloth wall hanging or such. This one is 18x24 and is quite handy for these tasks.